Adam Clarke on Exodus 20:22

I have talked with you from heaven - Though God manifested himself by the fire, the lightning, the earthquake, the thick darkness, etc., yet the ten words, or commandments were probably uttered from the higher regions of the air, which would be an additional proof to the people that there was no imposture in this case; for though strange appearances and voices might be counterfeited on earth, as was often, no doubt, done by the magicians of Egypt; yet it would be utterly impossible to represent a voice, in a long continued series of instruction, as proceeding from heaven itself, or the higher regions of the atmosphere. This, with the earthquake and repeated thunders, (see on Exo 20:18 (note)), would put the reality of this whole procedure beyond all doubt; and this enabled Moses, Deu 5:26, to make such an appeal to the people on a fact incontrovertible and of infinite importance, that God had indeed talked with them face to face.